A disk drive typically includes storage media, a read/write head and a head position encoder. The storage media is a platter that contains a large number of closely spaced concentric tracks. The head is positioned at a specific location over a track and instructed to read or write information on the media. In order to achieve high density storage, the tracks are located very close to one another and the placement of the head is very important. Head position encoders are known in the art and are roughly categorized into two types: electromechanical encoders and optical encoders. Electromechanical encoders generally sense current through magnetic coils to determine the head position. Optical encoders generally sense the application of light on a surface to determine the head position.
Known optical head position encoders are described in U.S. Pat. Nos. 4,703,176 and 5,084,791, incorporated herein by reference. The '176 patent describes a polyphase optical position encoder using a movable glass scale with alternating equally dimensioned opaque and translucent regions extending generally perpendicular to the locus of scale movement. A light source penetrates the scale and directs light onto a pair of photodetector areas symmetrically disposed about an axis of rotation of the disk drive head. The photodetectors are responsive to the light source and to the relative position of the scale. The photodetectors provide a quadrature-phase signal to a processor that is used to governs the movement of the head.
The '791 patent is similar to the '176 patent and includes a temperature detector. A processor is configured to determine the thermal expansion of the storage media based on a linear approximation and to adjust the head position based on the approximation of the thermal expansion. This technique is processor intensive and presumes that the thermal expansion is linear.
Accordingly, a limitation of existing disk drive head position encoders is that the temperature of the disk drive causes misalignment with the head and the recording tracks because of the thermal expansion of the storage media, and known compensation techniques for the expansion is performed by relatively complicated procedures of measuring the temperature and then estimating the position change. These compensation techniques also require additional hardware or firmware. Another limitation of existing disk drive head position encoders is that they determine position of the head based on a quadtrature-phase detection technique that does not provide a high degree accuracy.
What is needed is a disk drive head position encoder that is very accurate in position and that compensates for thermal expansion of the storage media in order to provide an accurate position of the head over the recording tracks of the storage media.